1. Field of the Invention
This invention relates to an automatically switching circuit for switching the sensitivity with which a call signal is detected in the portable handset of a cordless telephone from a high to low level or vice versa. Specifically, this invention relates to an automatic sensitivity switching circuit capable of setting the sensitivity at a relatively high level in the remote standby situation in which the portable handset is at a distance from the base unit, and of setting the sensitivity at a relatively low level when the portable handset is resting on the base unit.
2. Description of the Prior Art
As is well known, a cordless telephone comprises a base unit connected to the exchange line as is the case with an ordinary automatic telephone body, and a portable handset, which corresponds to the telephone receiver of the ordinary telephone, but can be separated from the base unit for the user to carry and use at a desired place as a wireless telephone receiver.
If the handset is not to be used for a relatively long period, as for instance at night, it rests on the base unit, whereby the input terminals of the handset are brought in contact with the output terminal of the base unit, thereby automatically charging a secondary battery contained in the handset.
In use, the user carries the handset to a desired place remote from the base unit, and then the handset is put in its standby situation in which the handset is ready to detect a call signal from the base unit by wireless connection. With a view to avoiding an undesirable situation of the handset not being able to pick up the call signal from the base unit so that it will not ring, the handset is usually designed to have sufficiently high sensitivity.
The high sensitivity on the side of the hadset is liable to cause erroneous ringing of the handset under the influence of undesired noise signals of radio frequency. Though such erroneous ringing may be permissible in the daytime when the level of ambient noise is high, it cannot be put up with at night after the user or others in the house have gone to bed.